The demand for high quality basestocks for formulation into engine oils and other lubricating needs is increasing due to heightened environmental concerns. Basestocks quality is being impacted by demands for basestocks that meet Group II or Group III requirements. Thus there is pressure for producing basestocks that meet the requirements of viscosity index (VI), viscosity, pour point and/or volatility imposed by governmental regulations and original equipment manufacturers. The ability of solvent refining alone to economically meet these increased demands for higher basestock quality is limited. Even with the use of additives, formulated oils require higher basestock quality to meet the demands of modern engines. Also, the supply of crudes that are rich in paraffins, is limited.
Catalytic dewaxing has developed as an alternative to solvent based methods for producing high quality basestocks. Dewaxing catalysts function by two different mechanisms: those catalysts which function primarily by isomerization and those catalysts which function primarily by hydrocracking. There are few, if any, dewaxing catalysts with the ability to function solely by one mechanism to the exclusion of the other. Dewaxing by hydrocracking can be done with relatively low quality feedstocks. However, these feeds typically require more severe reaction conditions to achieve target basestock quality and this leads to lower basestock yields and further processing steps to mitigate undesirable species formed by hydrocracking.
Dewaxing catalysts which function primarily by isomerization convert waxy molecules into branched chain molecules. Branched chain molecules can have desirable properties with regard to VI and pour point. ZSM-48 is an example of such a dewaxing catalyst. As noted in U.S. Pat. No. 5,075,269, ZSM-48 is prepared using diquaternary ammonium compounds as directing agents. Both the directing agent and the silica-alumina ratio can influence crystal morphology, although the choice of directing agent is the greater factor. When using a diamine or tetraamine directing agent, rod- or needle-like crystals are produced. At high silica:alumina ratios using a diquaternary ammonium directing agent, the ZSM-48 produced has a platelet morphology. As the silica:alumina ratio is lowered using the preparative techniques described in U.S. Pat. No. 5,075,269 or U.S. Pat. No. 6,923,949, crystal purity becomes an increasing problem as competing crystalline forms other than ZSM-48 are produced, or the ZSM-48 contains heterostructural zeolite seeds.
It is known that crystal morphology can affect catalyst behavior, especially with regard to catalyst activity and stability. Also, it is generally desirable to have a small crystallite size as smaller crystals likewise favor higher activity and stability due to greater surface area for given amount of catalyst.
It would be highly advantageous to have ZSM-48 crystals that could be made with high purity and that would have high activity when used as a catalyst while exhibiting a favorable morphology.